The present invention relates, in general, to a series of pendant boiler tubes fitted with shields for protection from sootblower erosion, and to a rigid structure used to maintain these pendant boiler tubes in parallel alignment in a predetermined spaced-apart configuration. More particularly, it relates to a split ring casting structured to cooperate with the tube shields in protecting the pendant boiler tubes from sootblower erosion.
Today's boiler systems are designed for long-term operational availability and high efficiencies. Part of the efficiency improvement is due to higher operating pressures and steam temperatures. From an efficiency point of view, boilers are designed to maximize the steam output with a minimum of fuel expended. This is accomplished through the maximization of the heat rate, i.e., fuel consumption, residence time in the heat transfer sections of the boiler, and maximization of the heat contact surfaces. However, buildup, slagging, and fouling of the boiler tubes will increase the insulation effect on the tubes, which does not allow for effective thermal transfer. This requires more fuel to maintain an adequate steam output. It also causes higher velocity through the heat transfer tube sections. This, in essence, reduces the residence time of the convective heat against the heat transfer surfaces which deteriorates the thermal transfer efficiency and requires more fuel to maintain the desired steam output.
Cleaning highly heated surfaces, such as the heat exchange surfaces of a boiler has commonly been performed by mechanical devices generally known as sootblowers which are used for on-line removal of fouling deposits from the boiler tube surfaces on a periodic basis. Sootblowers typically employ saturated steam, superheated steam, compressed air, water, or a combination thereof, as a blowing medium which is directed through a nozzle against encrustations of ash, slag, scale, and other fouling materials that are deposited on the heat exchange surfaces. Sootblowers of the retracting variety employ a long lance tube which is periodically advanced into and withdrawn from the boiler through a wall port, and is simultaneously rotated such that one or more blowing medium nozzles at the end of the lance tube project jets of blowing medium tracing helical paths.
Experience has shown that boiler tubes whose outer surfaces are subjected to impact by the high velocity and abrasive blowing medium suffer from erosion and wear. The problem of heat exchanger surface deterioration has been particularly severe in connection with cleaning the rigidly held tube bundles such as those made up of pendant boiler tubes found in large scale boilers. Since the pendant tubes are rigidly held, they cannot readily distort in response to the temperature induced shrinkage and expansion occurring during a cleaning cycle. Difficulties are also present in an effort to produce adequate cleaning performance while avoiding thermal overstressing since the heat exchanger tube surfaces to be cleaned are of varying distance from the lance tube nozzle and therefore a varying speed of blowing medium jet progression across the heat exchanger surfaces occurs. Areas of slow progression may receive excessive quantities of sootblowing medium as compared to the amount required for effective cleaning. Thus, physical deterioration of the heat exchanger surfaces may occur where the tubes are over-cleaned in this manner. Such degradation of the tubular heat exchange surfaces of a boiler can produce catastrophic failures and a significant financial loss for the boiler operator.
Accordingly, a protective device in the form of a shield is provided to prevent direct impingement of the outer surfaces of the tubes by the sootblower blowing medium while allowing the tubes to be cleaned of ash, slag, scale and other fouling deposits. The shield is normally comprised of an axially elongated member of arcuate cross section sized to fit over the outer surface of the tube to protect the portion of the tube which is impacted by the cleaning medium.
The described shield works well in protecting the outer surfaces of the tubes from the high velocity and abrasive blowing medium, but a problem arises when it is used with vertically elongated tubes such as those forming pendant heat transfer surfaces, located in the boiler furnace and convection pass, and referred to in the industry as superheaters and reheaters whose respective inlet and outlet headers and major supports are housed in a section referred to in the industry as the penthouse, the latter being situated above the furnace and convection pass roof line. The pendant loops of these tubular heat transfer surfaces support themselves in simple tension and are subjected to stresses due to differences in expansion between the different loops since their average temperatures are different because the fluid flowing along the tubes from the inlet to the outlet header is being heated. Therefore, it is desirable and necessary to provide split ring castings to maintain the pendant tubes in parallel alignment and spaced with respect to each other.
Referring to the prior art as illustrated in FIGS. 1 and 2, wherein like reference numerals denote like elements, there is shown a row of vertical lengths of essentially parallel boiler tubes 10 which are kept in alignment and spaced from each other by a rigid structure known in the industry as a split ring casting which is comprised of two halves 14 that are shaped with arcuate or semicircular grooves to fit around portions of the boiler tubes 10. The two halves are drawn together and clamped or fastened around the boiler tubes 10 by a cross-bar 18 to maintain the pendant tubes 10 in parallel alignment and spaced with respect to each other. A retainer shield 20 conforming to the external dimensions of the front end of the split ring casting is welded thereto as indicated at weld area 22, shown in FIG. 1. The boiler tubes 10 are fitted with semi-cylindrical shaped tube shields 24 for protection against the abrasive impingement of the high velocity fluid cleaning medium being ejected from sootblower nozzles, not shown. The tube shields are spaced from each other and cooperate with the outer surface of boiler tube 10 to form a recess 28 therebetween, as shown in FIG. 2. A serious problem has been encountered with this prior art arrangement due to the difference in thermal expansion of the tube shields 24 relative to the boiler tubes 10 at high boiler operating temperatures, that has resulted in the gaps 26 being formed between the tube shields 24 and the split ring casting 12 thereby exposing a portion of the outer surface of boiler tubes 10 to the abrasive impact of the high velocity sootblower fluid cleaning medium.
The aforementioned problem occurs in the unprotected tube area existing between the adjacent end faces of the tube shield 24 and the split ring casting 12. Efforts at structurally bringing these end faces together and eliminating any gaps 26 therebetween have met with failure as a result of the difference in thermal expansion of the tube shield 24 relative to the protected tube 10 at high boiler operating temperatures. Experience has shown that the gap 26 existing between the adjacent end faces of the tube shield 24 and the split ring casting 12 is one of the most vulnerable areas to sootblower tube erosion due to flow disturbances created around the split ring casting 12.
Accordingly, there is a need for a split ring casting structured to cooperate with the adjacent tube shields to insure that there are no boiler tube outer surface areas left unprotected from the abrasive impingement of the high velocity sootblower blowing medium.